Delay blasting initiators or detonators are well known in the art and normally consist of a metal or plastic shell or tube, closed at one end and containing a base charge of a secondary explosive, such as pentaerythritol tetranitrate (PETN), and a priming charge of a primary explosive such as lead azide located immediately above the base charge. A delay element is placed above the priming charge and an ignition charge is located above the delay element. The delay element introduces a time lag between the activation of the ignition charge and the detonation of the base charge. The ignition charge is activated electrically in an electric detonator and by means of energy provided by a detonating cord or shock tube in a non-electric detonator.
In multiple charge blasting operations, a number of closely spaced explosive-charged boreholes are advantageously detonated in a planned sequence employing mil-second (MS) delay blasting detonators. Use of such split-second techniques results in substantially improved blasting results in terms of improved fragmentation, reduced vibration and backbreak and minimized cut-offs. Briefly described, in split-second blasting, a single charged borehole or a row of charged holes is detonated at one point in time, a second adjacent charged hole or row of charged holes is detonated at a later mil-second interval, a third charged row at a further short delayed interval, etc. The delay between each detonation is achieved by providing blasting detonators having a built-in delay feature, the delays ranging from about 10 MS to about 9000 MS.
A problem which has persisted in the use of split-second delay blasting techniques has been the inadvertent, premature detonation of blasting detonators in nearby holes caused by shock transmitted through the terrain from an earlier detonated charge. When this occurs, the carefully planned sequence of delay blasting is upset resulting in unsatisfactory blasting results.